I am working on a program - it should be simple - on a Linux OS using NASM and x86 Intel Assembly Syntax.
The problem I am having is that I cannot create a working loop for my program:
section .data
hello: db 'Loop started.', 0Ah ;string tells the user of start
sLength: equ $-hello ;length of string
notDone: db 'Loop not finished.', 0Ah ;string to tell user of continue
nDLength: equ $-notDone ;length of string
done: db 'The loop has finished', 0Ah ;string tells user of end
dLength: equ $-done ;length of string
section .text
global _start:
_start:
jmp welcome ;jump to label "welcome"
mov ecx, 0 ;number used for loop index
jmp loop ;jump to label "loop"
jmp theend ;jump to the last label
welcome:
mov eax, 4
mov ebx, 1
mov ecx, hello
mov edx, sLength
int 80 ;prints out the string in "hello"
loop:
push ecx ;put ecx on the stack so its value isn't lost
mov eax, 4
mov ebx, 1
mov ecx, notDone
mov edx, nDLength
int 80 ;prints out that the loop isn't finished
pop ecx ;restore value
add ecx, 1 ;add one to ecx's value
cmp ecx, 10
jl loop ;if the value is not ten or more, repeat
theend:
;loop for printing out the "done" string
I am getting the first string printed, one "Not done" and the last string printed; I am missing nine more "Not Done"s! Does anyone have any idea as to why I am losing my value for the ecx register?
Thank you.
_start:
jmp welcome
This means all the code below the JMP is not executed, especially the mov ecx,0 (which should be xor ecx,ecx for a shorter instruction)
Don't start with a jump, start with some code. A JMP is a jump, it's not going back after you've jumped, it just continues the execution.
So after jumping to Welcome:, you go directly to Loop:, thus missing the ecx=0 code.
cmp ecx, 10
jl loop
ECX is not 0, it definitely is greater than 10h, so the loop is not taken.
Try this:
_start:
mov eax, 4
mov ebx, 1
mov ecx, hello
mov edx, sLength
int 80 ;prints out the string in "hello"
xor ecx,ecx ;ecx = 0
loop:
push ecx ;save loop index
mov eax, 4
mov ebx, 1
mov ecx, notDone
mov edx, nDLength
int 80 ;prints out that the loop isn't finished
pop ecx ;get loop index back in ECX
add ecx, 1 ;add one to ecx's value
cmp ecx, 10
jl loop ;if the value is not ten or more, repeat
theend:
You are setting the loop register ecx initial value to the address of "hello", and not 0:
jmp welcome
(mov ecx, 0) ;number used for loop index <- jumped over
...
welcome:
...
mov ecx, hello <- setting
int 80 <- ecx
...
loop:
push ecx ;put ecx on the stack so its value isn't lost
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